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Our future, our universe, and other weighty topics

Sunday, June 1, 2014

The Faint Young Sun Paradox: An Unsolved Mystery

The origin of life seems to
require liquid water. Geological evidence indicates that our planet
was warm enough for liquid water to have existed about 3.5 billion
years ago, when the first earthly life appeared. But models of solar
evolution lead scientists to conclude that the sun gave off much less
heat billions of years ago. Judging only from the sun's evolution,
our planet should have been completely frozen three billion years
ago. This discrepancy is known as the faint young sun paradox.

Below is a diagram from a
scientific paper by Shani and Shtanov discussing the faint young sun
paradox. As you can see from the diagram, if we assume (for the sake
of simplicity) that our planet has had its current atmosphere for the
past 3 billion years, then our entire planet should have been frozen
until about 1.7 billion years ago.

Scientists have advanced some
explanations for the faint young sun paradox, but none of them is
problem-free. One explanation is that billions of years ago our
planet's atmosphere was vastly different. It could have been that
billions of years ago our atmosphere had vastly more carbon dioxide and
methane. That would have produced a greenhouse effect many times
greater than the greenhouse effect currently produced by man-made
pollutants.

But there are some problems
with this theory. There would have needed to be almost
ridiculous amounts of carbon dioxide and methane in the atmosphere
for the greenhouse effect to have been sufficient to explain the
paradox. For example, in this link there's a discussion of some
scientists hinting that they have solved the faint young sun paradox
through a greenhouse gas explanation. But they imagine carbon dioxide
levels of 20,000 parts per million – fifty times greater than
today's level. Why are such high levels of
greenhouse gases implausible? A good explanation is given by this
scientific paper by Shani and Shtanov:

While
CO2 [carbon dioxide] can be removed from the atmosphere by
bacteria and plants during photosynthesis and also by the weathering
of rock, the enormous concentrations referred to earlier seem
difficult to account for. Indeed, large amounts of CO2 in
the early atmosphere would have led to the formation of the iron
carbonate based compound siderite
(FeCO3). However, analysis of billion year old paleosols
challenges this picture by finding no FeCO3. Found instead
are iron silicates which support a much more moderate presence of CO2
in the early atmosphere. Moreover, it would be difficult to make CO2
disappear almost entirely from the Earth’s atmosphere since much of
it, after reacting with rocks and being used by shell-forming
organisms to form CaCO3 shells, gets deposited on the
ocean floor and makes its way back into the atmosphere via volcanic
activity and plate tectonics . The presence of other greenhouse gases
is equally problematic.

The paper makes the point
that if you had once had fantastically high levels of carbon dioxide
(CO2) in the atmosphere, and that
had slowly disappeared, much of that carbon dioxide would have been
stored in what are called carbon dioxide “sinks.” But over the
eons the carbon dioxide in those sinks would have been slowly
re-released by geological processes – which would have left us with
more carbon dioxide in the atmosphere than we have today.

What solution do Shani and
Shtanov suggest for the faint young sun paradox? They suggest a
varying gravitational constant. The gravitational constant is the
fundamental constant that indicates the strength of gravitation
throughout the universe. It is one of several fundamental constants
that are crucially important in determining the behavior of the sun.
The authors suggest that if the gravitational constant had varied by
2 percent over the past few billion years, that could explain the
faint young sun paradox.

This explanation can be
called something of a “Hail Mary pass.” Scientists don't quite
have a rule of “if all else fails, suggest the laws of nature have
changed,” but assumptions about changing laws of nature or changing
physical constants are generally regarded as a kind of last resort.
This paper by Pitjeva and Pitjev claims to set a limit on changes in
the gravitational constant. The paper (based on spacecraft
observations) concludes that the gravitational constant changes by
less than 1 part in 20,000,000,000,000 per year. This conclusion is hugely
inconsistent with the hypothesis of Shani and Shtanov that the faint
sun paradox can be explained by assuming a 2 percent variation of the
gravitational constant. If Pitjeva and Pitjev are correct, the
gravitational constant could not have varied by more than 1 part in
10000 during the past five billion years, a limit which rules out a 2
parts in 100 (2 percent) variation as imagined by Shani and Shtanov.

That apparently shoots down a
varying gravitational constant as an explanation for the faint young
sun paradox. But what about other fundamental physical constants that the sun's
behavior depends on, such as the fine-structure constant? That's not
an area scientists prefer to be fiddling with, for two reasons.
First, it has already been concluded that the sun is extremely
sensitive to changes in the fine-structure constant, and that if it
were only the tiniest bit different, the sun would be some other type
of star such as a red dwarf or a blue giant. For example, this paper by a physicist finds that changing the fine-structure constant by 2 parts in 1000 would cause the earth to freeze, and changing the fine-structure constant the other way by only a few percent would cause the earth's water to boil. Given this extreme fine-tuning
of the fine-structure constant, do we really want to imagine some
other fine-tuning, some just-right slow change in the fine structure
constant to resolve the faint young sun paradox? Also, a study based
on earthly isotopes indicates that the fine-structure constant is
totally invariant over the eons, varying by less than 1 part in
10,000,000,000,000,000 per year.

So where does that leave us
in regard to the faint young sun paradox? We are left with an
unsolved mystery. Somehow our planet was warm billions of years ago,
but we really don't know why it wasn't completely frozen. Considering the matter, we should be
humbled, and remember how fragmentary and incomplete our knowledge of
nature is.

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All posts on this blog are authored by Mark Mahin, and are protected by copyright. Copyright 2013-2014 by Mark Mahin. All rights reserved. Any resemblance between any fictional character and any real person is purely coincidental.